Did the diff come out of a car that was involved in a big frontal wreck? If you just got the diff from a dismantler, and it came from a car that was wrecked in the front, I bet it was cracked when you put it in.

The only way those things break is from big frontal impacts, or slamming the PPF over curbs over and over again.

FYI, the 2nd gen N/A FC housings bolt right up and don't have that little notch. You just have to use a Miata bushing set.